1. Field of the Invention
The present invention relates generally to a magneto-optical recording medium to/from which information is recorded/reproduced with a laser beam by utilizing magneto-optical effects, and to a method for reproducing information recorded in this medium.
2. Related Background Art
Technologies have been developed vigorously relating to a rewritable high-density recording medium in which magnetic microdomains are recorded in a magnetic thin film by utilizing the thermal energy of a laser beam and from which signals are reproduced by utilizing magneto-optical effects. In such a medium, reproduction characteristics are impaired in the case where diameters and intervals of recording bits functioning as recording magnetic domains decrease relative to a beam diameter of a light beam converged onto the medium. This is because a beam spot formed by converging the light beam onto a target recording bit also falls on adjacent recording bits.
Therefore, the super-resolution reproducing method, the reproducing method in which magnetic domains are enlarged so as to be reproduced (hereinafter referred to as magnetic-domain enlarging-reproducing method) by utilizing the domain wall movement, etc. have been proposed. The following description will depict the magnetic-domain enlarging-reproducing method disclosed by JP 6(1994)-290496A, while referring to FIG. 8. According to the magnetic-domain enlarging-reproducing method, amplitudes of reproduction signals are widened by enlarging-recording magnetic microdomains in a recording layer 83. It should be noted that arrows in the drawing indicate directions of sub-lattice magnetizations of transition metals. However, a part 89 in an intermediate layer 82 loses the magnetic order since it is heated to a Curie temperature or above.
While the magneto-optical recording medium (disk) is irradiated with a laser beam 80, the disk is moved rightward in the drawing. Then, a position at which a film temperature has a maximum value is behind the center of a beam spot in a beam spot moving direction (leftward direction in the drawing). A domain-wall-energy density "sgr"1 in a magnetic-domain enlarging layer 81 usually decreases as the temperature rises. Therefore, if there is a temperature gradient, the magnetic-wall-energy density "sgr"1 is lower on the high temperature side. A force F1 expressed by the following equation is applied to domain walls in respective layers that are present at a position x on the medium:
F1=xe2x88x92d"sgr"1/dx
The force F1 is applied so as to move the domain wall toward a portion having a lower domain-wall-energy density, that is, to the high temperature side. Therefore, in the magnetic-domain enlarging layer 81 having a lower domain-wall coercive force, a domain wall 88 is moved to the high temperature side by the force F1 in a region where the intermediate layer 82 reaches a Curie temperature thereof and the exchange coupling is broken. Herein, the movement of the domain wall is sufficiently quicker than the movement of the medium. Thus, the magnetic domain stored in the recording layer 83 is transferred and magnified to the magnetic-domain enlarging layer 81.
The foregoing conventional magnetic-domain enlarging-reproducing method, however, has a drawback in that magnetic domains in adjacent tracks inhibit smooth movement of a domain wall in the track extending direction. Therefore, a technique in which the domain wall movement in a radial direction is prevented by magnetic separation of a target track from the adjacent tracks has been proposed. Two main schemes are shown below:
(i) forming rectangular guide grooves on a substrate, so as to separate tracks with the grooves; and
(ii) annealing recording layers in adjacent tracks, so that the layers have in-plane magnetization.
However, in the scheme (i), films actually are formed in step-like portions thereby connecting magnetic layers with each other. Therefore, it is difficult to achieve complete magnetic separation. As to the scheme (ii), an industrially applicable scheme has not been known, and the annealing of recording layers of adjacent tracks is disadvantageous with a view to high-density recording.
Therefore, with the foregoing in mind, it is an object of the present invention to improve a magneto-optical recording medium to which the magnetic-domain enlarging-reproducing method is applied, or more specifically, to provide a magneto-optical recording medium in which magnetic influences from adjacent tracks are suppressed when information is reproduced by utilizing magnetic-domain enlargement.
A magneto-optical recording medium of the present invention includes a substrate, and a multilayer film formed on the substrate, and the multilayer film includes a first magnetic layer, a second magnetic layer, and a third magnetic layer. The second magnetic layer is interposed between the first and third magnetic layers and has a Curie temperature TC2 that is lower than a Curie temperature TC1 of the first magnetic layer and a Curie temperature TC3 of the third magnetic layer, and the third magnetic layer is a perpendicular magnetization film. In the medium, in at least a part of a temperature range lower than the Curie temperature TC2, the first magnetic layer is exchange-coupled with the second magnetic layer so as to be perpendicularly magnetized, and a magnetization of the third magnetic layer is transferred to the first magnetic layer via the second magnetic layer due to the exchange coupling. The second magnetic layer is in an in-plane magnetization state at room temperature, and makes a transition to a perpendicular magnetization state in a temperature range from a critical temperature TCR that is higher than room temperature to the Curie temperature TC2.
In the present specification, xe2x80x9croom temperaturexe2x80x9d indicates 20xc2x0 C.
A magneto-optical recording medium reproducing method of the present invention is a method for reproducing information from the foregoing magneto-optical recording medium. The method includes (i) irradiating the medium with a laser beam while the laser beam is moved with respect to the surface of the medium so as to form a masked region and a perpendicular magnetization region, wherein the masked region is a region heated to a temperature not lower than the Curie temperature TC2 of the second magnetic layer and not higher than the Curie temperature TC1 of the first magnetic layer and the Curie temperature TC3 of the third magnetic layer, and the perpendicular magnetization region is a region where the first magnetic layer is exchange-coupled with the second magnetic layer so as to be perpendicularly magnetized and a magnetization of the third magnetic layer is transferred to the first magnetic layer via the second magnetic layer due to the exchange coupling, wherein a domain wall of the first magnetic layer is moved from the perpendicular magnetization region to the masked region, thereby causing a magnetic domain in the perpendicular magnetization region to be enlarged, and (ii) detecting a change in a polarization plane of a reflected light of the light beam from the enlarged magnetic domain.